1. Field of the Invention
This invention relates to a multiprocessor computer and a program which can assign tasks to a plurality of processors, in particular, a multiprocessor computer and a program which can level the processor temperatures among processors and suppress occurrence of stop of processing due to overheating.
2. Description of the Related Art
In the field of controlling the lines in factories and plants, widely used are computers performing control in synchronization with real time with which the object to be controlled operates. Such computers are equipped with hardware and software designed to achieve real-time processing.
Real-time processing is used for not only control processes in factories but also business information management processes and electronic commerce processes, and video process in a multimedia processes.
As computers of this kind, used is a multiprocessor computer which simultaneously uses a plurality of processors, if a required processing power is higher than a processing power of a single processor.
In many multiprocessor computers, a plurality of processes are assigned to one processor to use not only real-time processing but also the processing capability of each processor effectively.
When performing a process, a multiprocessor computer divides the process into processing units, i.e., tasks, assigns processors of numbers necessary for executing respective tasks, and executes the task. If a task requires a plurality of processors, an execution unit to be processed on each processor may be called a thread. Further, if a task and threads forming the task are executed over a plurality of periods on a processor, each period may be called a job. For example, a task which requires two processors is periodically executed, the task is formed of two threads, the number of which is the same as the number of processors to be used, and each thread is periodically executed as a job.
If a plurality of tasks are executed in a multiprocessor computer, it is necessary to schedule to determine the execution order of the tasks. Between scheduling methods, there is a scheduling method called dynamic scheduling in which the execution order of a job is determined when the job is actually carried out on the processor.
In the meantime, assigning a job to a processor which actually execute the job is called “dispatching”. In the dynamic scheduling method, scheduling and dispatching are done at the same time.
Algorithms for performing dynamic scheduling include a priority scheduling algorithm that does scheduling on the basis of the priority given to each task. An algorithm that does not change the priority once given is called a fixed priority scheduling algorithm. An algorithm that dynamically changes priority is called a dynamic priority scheduling algorithm.
The priority scheduling algorithm includes a method of determining the priority of a task or a job on the basis of the parameter of the task. An EDF (Earliest Deadline First) scheduling algorithm is a dynamic scheduling algorithm that decides priority on the basis of the deadline of the task or job. In addition, the EDF scheduling algorithm is a real-time scheduling algorithm that makes a feasibility check of scheduling and can assure that all tasks can keep real-time constraints (for example, refer to J. W. S. Liu, Real-Time Systems, Prentice Hall, 2000).
A multiprocessor computer having the above structure has no particular problem under normal conditions. However, according to the inventers' study, such a multiprocessor computer performs scheduling and dispatching in consideration of only priority, processing performance, and real-time processing. Therefore, the heat emission becomes unlevel among the processors, and noise (the sound of a fan, etc) caused by cooling of the heated processors increases. In the end, stop of processing may occur due to overheating.